The Universal Mobile Telecommunications System (UMTS) specified by the third generation partnership project (3GPP) shall offer appealing multimedia services. When many users want to receive the same multimedia services at the same time, like news services, even high bandwidth UMTS radio resources might run into shortage. Efficiency of resource allocation can be improved when many radio channels transfer data in parallel to different users within the same radio cell. A resource and therefore cost efficient delivery of multimedia services to a large number of users in parallel is given by broadcast or multicast services. As an example, 3GPP defined a Multimedia Broadcast and Multicast Service (MBMS) which shares network resources, specifically radio resources, when transferring data to many users in parallel. MBMS is an Internet Protocol (IP) datacast type of service that can be offered via existing cellular networks, such as Global System for Mobile Communications (GSM) or UMTS networks. The infrastructure gives the possibility to use an uplink channel for interactions between the service and the user. This is however not a straight forward issue in usual broadcast networks, as for example conventional digital television is only a one-way (unidirectional) system.
In general, a broadcast service can be generalized to mean a unidirectional point-to-multipoint service in which data is transmitted from a single source to multiple terminals, e.g. user equipment (UE) in third generation terminology, in the associated broadcast service area. In other words, broadcast services can be called push-type services. On the other hand, a multicast service can be defined as a unidirectional point-to-multipoint service in which data is transmitted from a single source to a multicast group in the associated multicast service area. Only the users that are subscribed to the specific multicast service and have joined the multicast group associated with the service can receive the multicast services. As a difference, a broadcast service can be received without separate indication from the customers. In practice, multicast users need a return channel for the interaction procedures in order to be able to subscribe to the desired services.
MBMS thus can be defined as a unidirectional point-to-multipoint service in which data is transmitted from a single source entity to a group of users in a specific area. As its name indicates, MBMS has two modes in practice: broadcast mode and multicast mode.
In 3GPP a working assumption for the downlink multiple access technology has been taken and it has been agreed to use Orthogonal Frequency Division Multiple Access (OFDMA) based technology in the downlink. OFDM is also being used for systems like Digital Video Broadcast (DVB) and DVB-Handheld (DVB-H). The use of OFDM is well suited in DVB for receiving the same synchronized content from different synchronized transmitters to enable coherent combination of the content.
In UMTS Terrestrial Radio Access Network (UTRAN), MBMS supports two different combining schemes, soft combining for cases where the content is identical, and selection combining when the content is—at least partly—cell specific or not synchronized. In case broadcast services are going to use an OFDM carrier separate from that of the dedicated services, the common MBMS services can be broadcast synchronously in neighbouring or adjacent cells, so that data reception is also possible at cell edges.
However, some broadcast data may be cell specific, localized broadcast data which should be also received at the cell edge. If the neighbour or adjacent cells are broadcasting localized MBMS data independent of each other, it is highly probable that data reception is hardly possible at the cell edge except in case the transmitted data is the same in the neighbouring or adjacent cells. A terminal device in a typical network environment at the cell edge would receive signals from more than one cell and the outcome of the signal combination would be erroneous, as the OFDM receiver cannot cope with signals having different content and being sent in the same time or frequency resource. A normal radio resource management (RRM) functionality is not valid, as the base station does not know the location and path loss of each user for MBMS and consequently the regular interference control means for normal packet data, where different users are given different resources depending on whether they need large power level or not, is not applicable at all for the MBMS case.
One solution to this problem has been suggested in Wideband Code Division Multiple Access (WCDMA) based systems, where a Rake type receiver with cell specific scrambling is provided to demodulate rather easily control information having different content by allocating a finger to the cell and not using any combination of signals from neighbouring cells or multipath propagation.